1. Field of the Invention
The present invention relates to a behavior control of vehicles against a spin and/or a driftout, and more particularly, to a behavior control device of a vehicle for controlling the brake system of the vehicle, so as to apply a controlled braking to a selected one or ones of the wheels, to act against an increase of a deviation of the actual yaw rate detected by a yaw rate sensor relative to a standard yaw rate estimated form the steering angle and the vehicle speed, when the deviation increases beyond a threshold value determined therefor.
2. Description of the Prior Art
It is already known to control the behavior of a vehicle against a spin and/or a driftout by applying a controlled braking to a selected one or ones of the wheels, by watching a deviation of the actual yaw rate detected by a yaw rate sensor relative to a standard yaw rate estimated from the steering angle and the vehicle speed, so as to execute such a behavior control when the deviation increases beyond a threshold value determined therefor, as described in, for example, Japanese Patent Laid-open Publication 6-115418. Such a behavior control is substantially effective in suppressing a spin and/or a driftout of a vehicle, since the deviation of the actual yaw rate detected by a yaw rate sensor relative to the standard yaw rate estimated from the steering angle and the vehicle speed represents the tire grip condition around the grip limit.
However, the effectiveness of such a behavior control is based on a premise that the yaw rate sensor produces a correct signal according to the actual yaw rate. The yaw rate in the running operation of the vehicles is directly detectable by the so-called yaw rate sensors of various type, generally incorporating an angularly inertial element. By such a yaw rate sensors the yaw rate is directly detected at high responsiveness. Nevertheless, the commercially available yaw rate sensors are not yet highly stable in their performances, because it is very difficult as a matter of technique to measure instant values of a varying angular velocity.
On the other hand, it is possible to estimate the yaw rate of a vehicle, when it is a four-wheeled vehicle, from a comparison of the wheel rotation speed of a pair of left and right wheels, particularly a pair of driven, i.e. non-driving wheels. The wheel rotation speed can be detected easily at high accuracy. However, the estimation of the yaw rate according to this method is liable to an error due to a slippage of the wheels against the road surface.
It is also possible to estimate the yaw rate of a vehicle from the lateral acceleration of the vehicle body and the vehicle speed, detected by a lateral acceleration sensor and a vehicle speed sensor, respectively. The lateral acceleration sensors, also available in various types, are generally much more stable in their performance than the yaw rate sensors, because a linear acceleration, even varying, is detectable by a much simpler mechanism than a varying angular velocity. However, the estimation of the yaw rate of a vehicle from the lateral acceleration and the running speed thereof incorporates therein an error due to a lateral and a longitudinal slippage of the wheel against the road surface. Further, the estimation of the yaw rate from the lateral acceleration and the vehicle speed is bound with an inherent delay in its response.